1. Field of the Invention
The present invention relates to an actuator having a reciprocatable moving means, suitable for use particularly but not exclusively in a device for locking and unlocking automobile doors through the manipulation of a switch. Still more particularly, the invention is concerned with an actuator of magnet moving type.
2. Description of the Prior Art
Door locking devices adapted to lock and unlock automobile doors by means of an electric switch have been already installed on high grade automobiles. Various types of locking devices of the kind described have been proposed hitherto. A typical example of this device is disclosed in "Automotive Engineer's Handbook" (ed. by Society of Automotive Engineers of Japan).
This device has a rod attached to the shaft of an actuator and fixed to a hook provided in each door. The hook is adapted to be brought into and out of engagement with a hinge provided in the body of the automobile in accordance with the reciprocative movement of the shaft, thereby to lock and unlock the door.
A torsion bar and a spiral spring are attached to the hook so that there is one peak point where the shaft encounters the maximum load in its single stroke from the locking state to the unlocking state or vice versa. Once the shaft is moved to one of the full lock or full unlock states beyond the above-mentioned peak, it cannot be returned to the other state naturally.
Thus, the torsion bar and the spiral spring in combination provide a fail-safe system in the door locking mechanism. Usually, the distance of movement of the shaft until the shaft gets over the peak point, i.e. the distance between the end of each stroke and the peak point in the same stroke is about 2.about.4 mm which is less than a half of the stroke length.
In order that the shaft is moved beyond the peak point, the maximum thrust generated by the actuator has to be about 24.5 N or greater. It is also necessary that the maximum stroke has to be produced in the initial period of the movement of the shaft. Namely, it is necessary that the actuator has such stroke-thrust characteristics that the thrust takes the maximum value in the initial period of the stroke and the level of the thrust is gradually decreased as the travel of the shaft is increased.
There are various types of mechanism for actuating the shaft reciprocatingly. For instance, the aforementioned "Automotive Engineer's Handbook" shows a solenoid type actuator at FIG. 2-398, Section 16.2, Chapter 2. This solenoid type actuator, however, has the following disadvantages although it exhibits a good respondence. This actuator is usually composed of a movable member or a plunger, two separate solenoid coils spaced in the axial direction and surrounding the plunger concentrically, and a yoke apparatus accommodating the coils. These two coils are adapted to be energized alternately so that the plunger is moved in one and the other direction by the electromagnetic attracting force acting between the plunger and the coils. Thus, in the conventional solenoid type actuator, it is necessary to use two solenoid coils although only one of them is used in each stroke. In addition, each coil is required to produce a magnetomotive force large enough to actuate the plunger. This means that each coil has to have a large size. In addition, in this type of actuator, the thrust is increased as the travel of the plunger is increased and the plunger has to be stopped forcibly at the end of its stroke, so that a large impact is produced accompanying with a large noise. In order to absorb this noise, a noise absorbing member is attached to each end surface of the yoke member and/or each end surface of the plunger. In consequence, the stroke length for the production of the thrust is increased resulting in a reduced level of the thrust. The volume and weight of the actuator are also increased undesirably.
Solenoid type actuator having a moving magnet has been put into practical use already. For instance, the specification of U.S. Pat. No. 3,149,255 (Trench et al.) shows at FIG. 9 an electromagnetic motor having a similar construction to the actuator of the present invention. This electromagnetic motor, however, is intended specifically for use as swing motors for air pumps or for use as vibrators or the like apparatus adapted to be driven by commercial A.C. power, and is not to intended for the operation with D.C. power which is used for driving the actuator for an automobile door locking device. In addition, in this electromagnetic motor, the magnetic piece of the movable member is disposed in axial alignment with the magnetic gap formed between the movable member and the yoke apparatus, in order to make an efficient use of the magnetic flux of the permanent magnet.
Japanese Utility Model Laid-open No. 54317/1979 discloses an actuator having a reciprocatable movable member. This actuator also is intended for use in pumps, vibration machines or the like apparatus driven by A.C. power, and is adapted to produce a substantially constant thrust over its entire stroke.
As has been stated, the actuators proposed and used hitherto are still unsatisfactory as the actuator for an automobile door locking device.